1. Field of the Invention
The invention relates to an eddy current measuring sensor and an inspection method using this eddy current measuring sensor. More particularly, the invention relates to an eddy current measuring sensor that improves the inspection accuracy in eddy current inspection, and an inspection method using this eddy current measuring sensor.
2. Description of the Related Art
Steel that has been induction hardened (hereinafter simply referred to as “steel”), in which metal (conductive material) is hardened by high-frequency induction heating, is used for mechanical parts such as underbody parts and engine parts of tomobiles and motorcycles. With regards to the hardened layer depth of surface hardening (hereinafter also referred to as the “hardening depth”) and the hardness in the induction hardening of steel, the effective hardened layer depth and the total hardened layer depth are standardized. Therefore, it is necessary to measure and evaluate the hardening depth and the hardness to ensure the quality of the steel.
The hardening depth and the hardness of the steel is evaluated by cutting out a portion of the steel as a sample, and measuring the sectional strength with any one of a variety of hardness gauges such as a Vickers hardness tester. However, with this destructive inspection method, the steel used as the sample is discarded, which leads to an increase in material costs. Also, in addition to the inspections taking longer, 100% inspection in a line is difficult, so steel may end up being moved to the next process without defects that occur sporadically being detected.
Japanese Patent Application Publication No. 2009-31224 (JP-A-2009-31224) and Japanese Patent Application Publication No. 2009-47664 (JP-A-2009-47664) describe technology for measuring the hardening depth and hardness of steel using an eddy current type inspection that is a nondestructive inspection. In this eddy current type inspection, an alternating current magnetic field is generated by moving an exciting coil carrying alternating current close to the steel. This alternating current magnetic field produces an eddy current in the steel. A detection coil then detects an induction field induced by this eddy current. That is, this eddy current type inspection makes it possible to quantitatively measure the hardening depth and the hardness of steel with 100% inspection and in a short period of time, without discarding the any steel. This eddy current type inspection is also used for flaw detection testing to detect flaws such as cracks in the surface of the object being inspected, and foreign material discrimination testing to detect foreign material in the object being inspected, in addition to hardening depth/hardness measurement testing to measure the hardening depth and the hardness of steel described above.
With regards to the conductivity of steel, there is a difference between the base material and martensite that occurs in the hardened layer. Therefore, in the hardening depth/hardness measurement testing described above, if the steel is measured using an eddy current sensor, the voltage (i.e., the amplitude) detected by the detection coil changes as the hardening depth changes and the voltage detected by the detection coil decreases monotonically as the hardened layer depth increases, so the hardening depth of the steel can be measured using these phenomena.
For example, the technology described in JP-A-2009-31224 is configured to inspect the hardening depth of a shaft portion of a shaft component using an encircling coil (i.e., an annular coil). The encircling coil has a stronger magnetic field than a probe coil does, and the distance to the steel does not need to be precisely controlled, which makes it suitable for hardening depth/hardness measuring testing. However, the diameter of the inner circumference which is the measuring portion of the encircling coil is fixed, so the filling rate of the measured portion with respect to the encircling coil (i.e., the ratio of the cross-section area of the measured portion of the steel to the inner peripheral cross-section area of the encircling coil) changes depending on the outer diameter of the measured portion of the steel. The inspection accuracy of the eddy current type inspection decreases exponentially as the filling rate decreases. Therefore, with the related art, the inspection accuracy differs due to the outer diameter of the steel changing at each measured portion. Also, the steel that is the object being inspected needs to be inserted through the encircling coil, so the range of application of this technology is limited to shaft components with a substantially constant outer diameter. That is, the inspection is difficult to carry out on a component in which the outer diameter changes significantly, such as a crankshaft, for example.
The technology described in JP-A-2009-47664 described above is configured to measure the hardening depth of steel using a probe coil. With the hardening depth/hardness measurement testing, the ratio of detected signal components to noise components is lower than it is with either flaw detection testing or foreign material discrimination testing, so greater inspection accuracy can be obtained. However, the probe coil has a weaker magnetic field and the distance to the steel must be precisely controlled, so while it is suitable for flaw detection testing and foreign material discrimination testing, it is difficult to use for hardening depth/hardness measurement testing.
Further, with the probe coil according to the related art, the magnetic field in the work is unable to be widened or offset to one side, so it is difficult to appropriately control the spread and direction and the like of the magnetic field. Furthermore, in flaw detection testing with the probe coil as well, a so-called edge effect in which a flaw signal ends up getting buried in an edge signal at an end portion of the steel severely limits the inspectable range and the parts that can be inspected and the like, so the reduction of this edge effect in eddy current measurements using a probe coil has become an issue.